1. Field of the Invention
This invention relates to disk drives for computers and, more specifically, to spindle motors for "Winchester" hard disk drives.
2. Prior Art
Disk drive memory systems ("disk drives") have been used to store information for computers for many years. In disk drives, digital information is recorded on concentric memory tracks on magnetic disks. There are two basic kinds of disk drives: "floppy" disk drives and "hard" disk drives. In both kinds of disk drives, disks are rotatably mounted on a spindle. Read/write heads, generally located on pivoting arms, move radially over the surface of the disks to access different memory locations. There may be as many as 300 or more memory tracks per radial inch of a disk. To insure proper writing and reading of information, a read/write head must be accurately aligned with an appropriate track on a disk. Floppy disk drives store information on interchangeable, flexible and magnetic disks. Hard disks store information on rigid non-interchangeable, disks, commonly made of aluminum. Hard disks have a much higher storage density than floppy disks. Hard disks are often located within the housing of a computer and may consist of multiple rigid metal disks stacked on top of each other within the drive. The disks are spun at high speed by a motor to allow rapid writing and reading of information on the disk. Because of the high storage densities of hard disks, it is extremely important that distortions and misalignments of the disks, and of the disks in relation to the motor spindle, be minimized to allow accurate information exchange between the read/write head and the disk.
In most instances, hard disks are driven by an electric motor located below the spindle on which the hard disks are mounted. Spindle motors have also been disclosed that incorporate the drive motor within the spindle itself. Such an in-spindle motor is described in co-pending application Ser. No. 444,465 filed Nov. 24, 1982 and assigned to the assignee of the present invention. In-spindle motors require less space than external motors, allowing for the construction of compact disk drive assemblies. In in-spindle motors the spindle forms part of the actual motor. To produce the required electromagnetic characteristics, present designs for in-spindle motors have required the use of spindles made of steel or other ferromagnetic materials. Magnets are mounted to the spindle such that the spindle forms a flux path for the magnets. Such a flux path is necessary to produce the amount of torque required to spin the disk assembly.
The use of a steel spindle with aluminum disks, however, leads to distortion and misalignment of the disks due to the differing thermal expansion coefficients of steel and aluminum (the coefficient for aluminum is about 1.6 times that of steel). As the temperature rises, the spindle, made of steel, expands more slowly than the disks, made of aluminum. In the radial direction, when the temperature is reduced, the diameter of the spindle decreases more slowly than the inside diameter of the disks, creating an interference between the spindle and the mounting hole of the disk that can lead to radial displacement of the disks. Both of these effects can cause a shift in the position of the magnetic memory tracks, leading to potential misalignment between a disk and the corresponding read/write head. This misalignment limits the storage density of the disks by placing a limit on how closely memory tracks can be spaced.
To compensate for these thermal expansion effects, some prior art devices use spacers and rings made of materials that, in combination with the aluminum disks, attempt to produce the same net thermal expansion in the disk stack as in the spindle.